9-(Z)-RA is a versatile compound which is claimed to be useful for the treatment of numerous dermatological diseases as e.g. disclosed in WO 99/09969.
The EP-A 0 659739 discloses a process for the preparation of 9-(Z)-RA which is characterized by the Wittig-reaction of an alkyl β-formylcrotonate with a C15-triarylphosphonium salt in the presence of a base and by the subsequent saponification of the so formed retinoic acid ester with a base.
This process suffers from the disadvantage that it requires two steps involving a change of solvent for the saponification of the retinoic acid ester. Since the saponifaction needs quite drastic temperature conditions a significant formation of unwanted isomers has also been observed.